This invention relates to a radio frequency electrosurgery system and a method of operating an electrosurgical inset at UHF frequencies.
It is known to use a needle or narrow rod electrode for cutting tissue in monopolar electrosurgery at frequencies in the range of 300 kHz to 3 MHz. An electrosurgical signal in this frequency range is applied to the electrode, and the electrical current path is completed by conduction through tissue to an earthing plate secured to the patients body elsewhere. The voltage applied to the electrode must be sufficiently high to cause arcing and consequent thermal rupture so that tissue adjacent the needle is ablated or vaporised.
At lower power levels, coagulation of the tissue can be achieved, i.e. without arcing, due to thermal dissipation of energy in the tissue adjacent the electrode, However, with a narrow electrode as commonly used for tissue cut, desiccation of the tissue immediately adjacent the electrode and build-up of desiccated material on the electrode itself constitutes a high-impedance barrier to further coagulation. Spatula-shaped electrodes have been produced to overcome the difficulty in providing a dual-purpose electrode, i.e. one suitable for both cutting and coagulation. The designer""s intention is that the edge of the electrode is used for cutting, whereas the flat surface is used for coagulation. However, coagulation with such an electrode tends to be imprecise due to the size of the flat surface, with the result that a large thermal margin is produced.
It is an object of the invention to provide a means of achieving both tissue cutting and coagulation with a single electrode assembly.
According to this invention, there is provided an electrosurgery system comprising an electrosurgical generator, a feed structure and an electrode assembly, the electrode assembly having at least one active electrode and at least one adjacent return electrode each of which is coupled to the generator via the feed structure, wherein the generator and feed structure capable of delivering radio frequency (r.f.) power to the active and retain electrodes in lower and upper frequency ranges, the upper range containing frequencies at least three times the frequencies of the lower frequency range. The lower frequency range may extend from 100 kHz to 100 MHz preferably 300 kHz to 40 MHz, and the upper frequency range may extend from 300 MHz to 10 GHz, preferably above 1 GHz, with operating frequencies in the upper and lower ranges having a frequency ratio of 5:1 or greater. Typically, the generator is arranged such that the r.f. power delivered in the upper frequency range is at a fixed frequency which is at least ten times the frequency of power delivered in the lower frequency range. Indeed, a fixed frequency of 2.45 GHz in the upper frequency range is preferred.
The preferred system allows simultaneous delivery of lower and upper frequency range components to the electrodes to provide a combination of medium or low frequency tissue cutting, vaporisation or ablation together with coagulation of surrounding tissue to a degree dependent upon the amplitude of the component in the upper frequency range.
For tissue cutting, vaporisation or ablation the system preferably operates in a monopolar mode with a separate earthing electrode applied to the outside of the patient""s body, whilst coagulation occurs in a quasi-bipolar mode whereby the return current path in the upper frequency range runs from the tissue adjacent the operation site to the return electrode of the electrode assembly due to capacitive coupling. It will be understood that the system may allow selection of power delivery either in the lower frequency range or the upper frequency range depending upon the kind of treatment required. This selection may be performed manually by the surgeon or automatically in the manner to be described below. In addition, power may be supplied in both frequency ranges simultaneously to obtain a blended cutting and coagulation effect, the two components being linearly added or otherwise combined in a single signal feed structure.
In a particularly preferred embodiment of the invention, the generator includes a control circuit responsive to electrical load and operable to cause the delivered power to have a predominant frequency component in the lower frequency range when the load impedance is in an upper impedance range, and to have a predominant frequency component in the upper frequency range when the load impedance is in a lower impedance range. In this way, it is possible to cut, ablate or vaporise living tissue (i.e. causing cell rupture) with the lower frequency range component but also to bring about efficient coagulation when a very low load impedance is detected, indicating the presence of electrolytic fluid such as blood from a blood vessel, requiring coagulation. The system reverts to predominantly low frequency operation once the impedance has risen above a predetermined threshold following coagulation.
When electrical load impedance is used as the control stimulus, a signal representative of load impedance being compared with a reference signal, the reference signal may have different levels depending on whether the generator is to be switched from a predominant low frequency component to a predominant high frequency component or vice versa. In other words, different load impedance thresholds may be selected when operating in the lower frequency range or the upper frequency range respectively.
A composite signal having components from both frequency ranges may be produced by combining (e.g. adding) the signals from two generator stages, one operating in the region of, say, 1 MHz and the other operating at 2.45 GHz. Both generator stages may be in a single supply unit coupled to an electrosurgical instrument which consists of a handpiece mounting the electrode assembly so that, for instance, the two frequency components are fed from the supply unit to the handpiece by common delivery means such as a low loss flexible coaxial cable. Alternatively, the generator stage producing the UHF frequency component may be located in the handpiece to reduce transmission losses and radiated interference, the signal combination being performed within the handpiece as well.
For dual-purpose operation, i.e. cutting and coagulation, an electrode assembly having a needle-like active electrode is preferred.
Typically, the electrode assembly is at the distal end of a rigid or resilient coaxial feed forming the above-mentioned feed structure. To reduce extraneous UHF radiation, an isolating choke element in the form of a conductive quarter-wave stub or sleeve may be mounted to the outer supply conductor of the coaxial feed in the region of the distal end. As stated above, the active electrode may take the form of a rod or pin projecting from the coaxial feed distal end. The return electrode may be a conductive sleeve, plate or pad connected to the outer supply conductor at the feed distal end and extending proximally over the outer conductor but spaced from the latter so that the active electrode rod and the return electrode sleeve, plate or pad together form an axially oriented dipole at the operating frequency of the generator in the upper frequency range. Alternatively, the return electrode simply takes the form of a distal end portion of the feed outer conductor located distally of the choke. The return electrode may be covered with an electrically insulative layer in order that, when the active electrode is applied to tissue, the return electrode, being set back from the active electrode so as normally to be spaced from the tissue, acts as a capacitive element forming part of a capacitive return path between the treated tissue and the return supply conductor of the feed.
In an alternative embodiment in accordance with the invention, the electrode assembly includes a gas supply passage and the active electrode is located within the passage where it acts as a gas-ionising electrode. In this case, the active electrode acts as a low- to high-impedance transformer at the operating frequency of the generator in the upper frequency range, producing an intense electric field in the space between the distal end portion of the active electrode and the return electrode. Accordingly, when there is an ionisable gas in the passage, the major part of the power delivered to the electrode assembly in the upper frequency range is dissipated in the passage. In the lower frequency range no transforming effect occurs and the frequency component in the lower frequency range is, instead, delivered to the tissue to be treated by the ionised gas plasma which, in effect, acts as a monopolar gaseous electrode. Use of a UHF frequency component as a plasma generator and a lower frequency component for electrosurgery allows independent control of plasma generation and electrosurgical power delivery, thereby avoiding the disadvantage of known single r.f. source gas plasma electrosurgery devices. Typically, in such a prior device the ability of the source to deliver current through the plasma is severely hampered due to the requirement for high peak voltages when using low frequencies (i.e. typically, less than 1 MHz).